Apparatuses and devices, whose operation or function is largely based on generating strong, and, when indicated, also changing magnetic fields, are becoming more and more prevalent in the context of apparatuses that are used for manufacturing and maintenance purposes, or also in the medical field. This is also true of apparatuses, which, until a few years ago, were used exclusively for basic research purposes.
Particle accelerators for therapeutic purposes come to mind in this case, for example. Particle accelerators can be used, in particular, for purposes of tumor therapy. Even inoperable tumors, especially brain tumors, can be successfully treated with the aid of particle accelerators. These types of therapeutic particle accelerators not only use electrons as accelerated particles, but in recent times, ions (mostly carbon ions) as well. Not only are linear accelerators used as particle accelerators, but synchrotrons (accelerator rings) are as well.
Other examples of devices whose operation or function is largely based on generating strong, and, when indicated, also changing magnetic fields, are devices in the manufacturing sector, such as devices used in induction hardening, for example. The medical sector also includes other fields of application, such as NMR diagnostic imaging devices (NMR denoting “nuclear magnetic resonance”).
While the cost effectiveness of a system is rather a low priority in pure research applications, in the manufacturing, maintenance and medical sectors, applications increasingly require a most economic possible operation of the systems.
Magnets in use today have the inherent problem of reproducibility of the magnetic fields. This is particularly true of magnets that are required to generate a time-variant magnetic field. Moreover, particularly when working with magnetic fields that change frequently and relatively quickly over time, the problem arises that a specific setpoint magnetic field is to be reached as quickly as possible.
For example, to provide a most economic operation of a synchrotron possible, it is desirable to achieve, on average over time, a highest possible percentage of the time for emission of the particle beam. This can be achieved by reducing to the greatest degree possible the time percentages in which the synchrotron is in the remaining operating modes. To this end, the time periods used for injecting and accelerating the particle beam in the synchrotron are kept as short as possible. This, in turn, requires keeping the amount of time needed to build up (ramp up), reduce (ramp down) and stabilize the magnetic fields used for beam deflection (dipole magnets) and beam focusing (quadrupole magnets) as short as possible.
Presently, in the case of particle accelerators, the magnets are generally controlled by current sources which apply current to the magnets at a predefined (typically time-variant) current intensity. To this end, what are generally referred to as DCCT (denotes “direct current to current transformers”) have proven effective over the years. To ascertain the relationship between the current, which flows through the magnetic coils, and the magnetic field, laboratory measurements are first performed on the magnets. For each applied current intensity, the magnetic field prevailing in the magnet is measured. To this end, a suitable probe—in practice, mostly an NMR or a Hall-effect sensor—is used. The measurement data obtained are used to generate a characteristic map that is subsequently used to control the magnet. Since the magnetic field also changes within the magnet as a function of location, additional laboratory measurements are necessary to determine the location dependency.
The remanence of magnetic materials complicates the process of generating characteristic maps within the framework of laboratory measurements. Virtually without exception, the magnets (dipole magnets and quadrupole magnets) currently used in accelerator systems are electromagnets that have normal-conducting current coils having cores and yokes of soft magnetic material (mostly bonded together iron sheets). When the electromagnet generates a magnetic field in response to the application of electric current, the soft magnetic material is automatically magnetized. Even when the electric current is subsequently switched off, a residual magnetization of the soft magnetic material remains, which is generally known as remanence. Thus, a magnetic field also remains in the magnetic gap of the electromagnet. The remanence field strength is a static property and does not decay over time. In this context, the intensity of the remanence field strength is not only dependent on the materials and the configuration of the electromagnets, but also on the magnetization prehistory of the electromagnet, respectively of the soft magnetic material. Thus, a consistently same magnetic field strength cannot necessarily have a specific, constant magnet current value In assigned thereto.
To be able to deduce the magnetic field strength generated by the electromagnet from a specific magnet current value In, it is necessary to observe an established procedure to arrive at the magnet current value In. A defined magnetization prehistory is thereby obtained. This is described as “conditioning.” Normal conditioning processes provide for controlling the magnet to its maximum value (and thus for driving the soft magnetic materials into saturation), and then subsequently reducing it to zero current, in order, from there, to approach the setting value. Following the conditioning process, the reproducibility of the magnetic field is typically better than 10-4 T (at maximum field strengths of typically 1.5 T to 2 T). For beam guidance magnets, the maximum remanence field Br is typically between ±1·10-3 T and ±3·10-3 T. For magnets having unipolar operation, the maximum remanence field Br is typically lower and is within the range of approximately ±2·10-4 T to ±4·10-4 T. The characteristic maps determined in the laboratory are based on this type of defined conditioning.
In the field of particle accelerators, remanence means that the preceding acceleration cycle(s) fundamentally influence(s) the properties of the magnet(s) in the subsequent acceleration cycle. Since in the case of particle accelerators, magnetic field variations of typically 2·10-4 T to 4·10-4 T play a critical role in determining beam loss or unacceptable changes in the beam properties, the ambiguity between the magnetic current and the generated magnetic field due to remanence is generally not tolerable.
Therefore, in the case of particle accelerators, a conditioning of the accelerator magnets has already been provided. A typical conditioning process provides for using what is generally referred to as a “chimney.” In this case, at the end of a particle beam extraction cycle, the accelerator magnets are normally driven to saturation and are subsequently driven to zero current. This creates controlled initial conditions for the next acceleration cycle. The disadvantage is obvious: To ramp up the magnets, energy is consumed without deriving any actual benefit. Time is also required to ramp up the magnets to the saturation region (and to subsequently ramp them down). The required time period is all the greater, the lower the particle energy is in the preceding acceleration cycle.
Under certain circumstances, a conditioning can also be carried out using what are generally referred to as “training cycles.” To this end, a defined magnetization prehistory of the magnets is made available, whereby it is not necessarily required that the magnets be ramped up to the saturation region. Such an approach can be appropriate when it is only rarely necessary to alter the particle energy of the particle beam. However, when the required magnetic field sequences deviate from the conditioning cycles, the use of training cycles is de facto not possible. Since, in practice, up to five (sometimes even more) training cycles are required, too much energy and beam time would then be lost. However, in medical applications, in particular, the particle energy must be changed very frequently. This is particularly true of raster scanning methods used for treating tumors.
Another source of ambiguities in the relationship between the magnet current and the magnetic field are the dynamic effects in the magnets when they generate rapidly time-variant magnetic fields. This is primarily caused by eddy currents. Conditioning is not effective in countering dynamic deviations. However, since the dynamic deviations decay over time, the errors can be reduced by waiting. In the case of a typical synchrotron magnet having a yoke of bonded together iron sheets, that is ramped in one second from zero to its full field (typically within the range of 1.5 T to 2 T), the initial store of the static end field is up to ±3·10-3 T. Thus, the initial dynamic store is within the range of the remanence errors or is greater than the same. In the case of bonded sheets, the decay time constant is normally within the range of 0.3 seconds. However, in the case of magnets made of solid iron, the decay time constant can also be within the range of many seconds.